The de novo synthesis of short and medium chain saturated fatty acids from acetyl-CoA and malonyl-CoA is a complex process catalyzed by several enzyme activities. In most bacteria and in plants these activities are associated with discrete monofunctional polypeptides. In fungi and animals however, these activities are integrated into one or two multifunctional polypeptide chains. Fatty acids, in particular in the form of oils and fats, which are glycerol esters, play a major role in human nutrition because of their high energy content. Additionally, specific types of fatty acids (e.g., polyunsaturated fatty acids such as DHA) have a wide range of physiological effects. There is great interest in being able to manipulate specific types of fatty acids made in organisms that produce these fatty acids (e.g., in the form of oils and/or phospholipids).